There is known a tandem-type color laser printer including a plurality of photosensitive drums and developing rollers corresponding to the colors of yellow, magenta, cyan and black respectively. In the tandem-type color laser printer the toner images of each of the colors are formed substantially concurrently on each of the photosensitive drums by each of the developing rollers, and subsequently, each of the toner images is transferred on a recording paper or an intermediate transfer belt. Thus, a color image can be formed at substantially same speed as a black-and-white laser printer.
Incidentally, in such a tandem-type color printer, the photosensitive drums of each of the colors need to be positioned parallel to one another with accuracy. If the rotating shafts of the photosensitive drums are not parallel to one another, the toner images of the respective colors are displaced from one another, thereby causing color shift.
To prevent this displacement, there is a method of inserting the supporting shafts of the photosensitive drums into supporting holes that are formed accurately in side plates and that has diameters corresponding to the diameters of the supporting shafts. However, in this method, it is difficult that the plurality of photosensitive drums is concurrently inserted into the supporting holes. Therefore, Japanese Patent Application Publications No. 8-87152 and No. 2001-222201 disclose a method that one end portions of the supporting shafts are supported by two side plates. The supporting shafts are positioned in a horizontal direction by one side plate and in a vertical direction by the other side plate.
Moreover, in Japanese Utility Model Application Publication No. 7-36367, springs are provided to push the supporting shafts of the photosensitive drums in one direction, thereby clearing clearances between the supporting shafts of the photosensitive drums and the bearings thereof or gaps between the bearings and the side plates.